Conventional systems for the direct injection of fuel into the combustion chamber of an internal combustion engine are known. A distinction is made in this context between so-called stratified mode as a first operating mode, and so-called homogeneous mode as a second operating mode. Stratified mode is used in particular at lesser loads, while homogeneous mode is utilized when greater loads are being placed on the internal combustion engine. In stratified mode, fuel is injected during the compression phase of the internal combustion engine into the combustion chamber, specifically into the immediate vicinity of a spark plug therein. The result of this is that uniform distribution of the fuel in the combustion chamber can no longer occur. The advantage of stratified operation is that the prevailing lesser loads can be handled by the internal combustion engine with a very small mass of fuel. Greater loads cannot, however, be handled in stratified mode. In the homogeneous mode provided for greater loads of this kind, fuel is injected during the intake phase of the internal combustion engine, so that turbulence and thus mixing of the fuel in the combustion chamber can readily take place. To this extent, homogeneous mode corresponds approximately to the method of operation of internal combustion engines in which fuel is injected in conventional fashion into the intake manifold.
In both operating modes, i.e. in stratified mode and in homogeneous mode, the injection angle and injection time of the fuel being injected are controlled in closed-loop and/or open-loop fashion by a control device, as a function of a plurality of parameters, to a value that is optimum in terms of fuel economy, emissions reduction, and the like.
For starting, the internal combustion engine is operated in homogeneous mode. Fuel is thus injected into the combustion chamber during the intake phase. The injection angle and the injection time during which fuel is injected are calculated by the control device prior to the intake phase. This calculation is performed in real time, i.e. on the basis of the operating state of the internal combustion engine existing prior to the intake phase.
During starting, the internal combustion engine is accelerated very rapidly. The result of this can be that the previously calculated injection angle and/or the previously calculated injection time no longer correspond, at the time of the actual injection action, to the operating state of the internal combustion engine existing at that time.
It is thus possible, for example, for the injection time to have been calculated for a low engine speed, but for the engine speed then to increase, because of the acceleration during starting, in such a way that the injection time is now too long. The result of this can be that fuel is still being injected into the combustion chamber, and the injection valve is thus still open, while the injected fuel is already being ignited by the spark plug. This causes premature carbon deposition on the injection valve, and is thus undesirable.
It is furthermore possible, because of the increase in engine speed during starting, for the injection valve still to be open even when the compression generated by the piston is already greater than the injection pressure that is being exerted on the fuel being injected. The result of this is then that a fuel-air mixture can be blown back out of the combustion chamber into the injection valve and thus into the fuel conditioning system. This causes subsequent misfires and the like during operation of the internal combustion engine.
It is the object of the present invention to create a method for operating an internal combustion engine with which the internal combustion engine can be more easily started.